EFFECTS OF PISTON BOWL GEOMETRY ON COMBUSTION AND … · 2017-11-03 · Abstract The present paper...

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308

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Volume: 05 Issue: 07 | Jul-2016, Available @ http://ijret.esatjournals.org 81

EFFECTS OF PISTON BOWL GEOMETRY ON COMBUSTION AND

EMISSION CHARACTERISTICS ON DIESEL ENGINE: A CFD CASE

STUDY

Chetan S Bawankar1, Rajesh Gupta

2

1PG Research Scholar, Department of Mechanical Engineering, Maulana Azad National Institute of Technology,

Bhopal, India 2AssociateProfessor, Department of Mechanical Engineering, Maulana Azad National Institute of Technology,

Bhopal, India

Abstract The present paper investigates effect of piston bowl geometry on combustion and emissions. One of the important parameters

which affect the combustion in diesel engine is piston geometry. In this study four different piston geometries (HEMISPHERICAL,

OMEGA, SINGLE CURVED, DOUBLE CURVED) have been selected for comparative study. Numerical investigation has been

carried out using commercially available code AVL FIRE which is a finite volume based code .Automatic mesh generation feature

of code is utilized for modelling. For validation of numerical model, simulation results of baseline shape have been validated with

experimentally available results. Simulation has been carried out for various piston geometries while keeping parameters such as

speed, compression ratio and mass of fuel injected constant. The study has been carried out at full as well as half load .A

comparative study for in-cylinder pressure, rate of heat rejection, velocity and temperature field, turbulence kinetic energy has

been performed .Emission and performance parameters like SOOT, NO ,efficiency and BSFC have been studied . The results

indicated that omega shaped piston works best for the current set-up. As compared to baseline shape of hemisphere, it gives 12%

higher NO but 19% less SOOT. Also, the efficiency and BSFC are better for omega shaped piston.

Keywords- OMG, SC, DC, HEMI, piston bowl, combustion, emission, CFD, AVL, simulation

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1. INTRODUCTION

Global warming is the biggest threat faced by our mother

earth in the present era. UN climate change conference 2015

held at Paris was the most discussed and major step toward

this threat. A majority of the pollution across globe is caused

by exhaust emissions from automobiles. Hence a lot of

research is going on for the development of various

strategies, methodologies that would bring down the

emissions. Carbon monoxide, oxides of nitrogen and soot

are the main culprits responsible for deterioration of air

quality. Diesel engine due to its excellent portability and

performance remains a primary choice for powering

vehicles other stationary devices.

Various strategies have been developed over the time for

emission reductions. Alternate fuels, that is biofuels, and

blending is one of prominent field which plays significant

role in emission reductions (1-3,34). Other studies suggest

that fuel injection timing along with pulsed injection have

significant effects on the engine emissions (4-6). Studies

relating combustion in diesel engine states that swirling is a

major factor affecting and promoting complete combustion

which in turns reduces exhaust emissions(8). Intake pressure

too affects the combustion (4).In cylinder flow is primarily a

function of bowl geometry, turbulence kinetic energy and

pattern of mixing of air-fuel mixture(7). Exhaust gas

recirculation (EGR) is an after treatment methodology

concerning combustion and reduction of emissions (9).G.

Sivakumarhave used a chemically coated piston crown to

study the effects on performance and emission (33). Various

permutations and combinations of above factors and its

effects are being studied by scientists and engineers.

Piston bowl geometry is an important factor in the mixing of

air-fuel mixture in engine. Various combinations of it with

other parameters have been studied. Jaichandar S et al. has

studied effect of injection timing and combustion chamber

geometry fuelled with biodiesel on diesel engine(10). Saito

T et al have investigated effect of combustion chamber

geometry on diesel engine(11). An insight to in-cylinder

flow was provided by Payri F(12)et al by CFD modelling.

Computational study on HCCI engine has been carried out

with two different piston bowls by T. Karthikeya Sharma

(32). Rakopoulos CD et al have studied piston bowl

geometry and speed effects on a motored diesel engine

configuration(13). But very few studies have compared

different configuration of piston geometry(14,28,29).

In this study we have selected three additional piston shapes

and comparison among all has been done based on various

parameters. The comparative study will give us an insight

into the effects of piston shapes on parameter and help in

finding out such factors which will be critical to design of

piston bowl geometry. MuruganSivalingamhave done the

theoretical modelling and its validation by comparing


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combustion parameters (31).Similarly, computer modelling

is another way out for our problem. In this study,

comparison and analysis will be done using computer

simulation. There are many commercially available codes in

the market of computer simulation. We have chosen a

control volume based CFD code AVL FIRE for our

simulation and analysis. The detailed methodology of

current study is described further.

1.1 Methodology of study

Below is the description of methodology adopted for this

study. The results for half and full load diesel engine were

already available. The engine specification has been

mentioned in table 1. Now with the help of AVL FIRE CFD

code we have prepared the numerical model using suitable

initial and boundary conditions. This numerical model has

been validated with experimental pressure and heat release

profile which is a general trend in validation study (15,30).

Since it has already been observed that piston bowl plays an

important role in the combustion process and hence affects

performance and emissions in diesel engine, we will be

changing piston shape and doing the simulation with new

configuration, successively. All the other parameters like

compression ratio, injection angle, injection pressure etc.

will remain constant. We have selected three more piston

geometries; named as omega (OMG) shaped, single curved

(SC) and double curved (DC). The baseline piston geometry

is hemispherical (HEMI). Fig 1 shows pictorial

representation of various shapes. Practical feasibility and a

little bit of imagination is the motivation behind selecting

these particular shapes. OMG and SC piston are already in

use and widely studied. The curvature of SC piston is altered

and one more piston shape is added to the study resulting in

DC piston shape. This is a product of curiosity and

imagination. Few studies have considered this type of shape

(16). Since the wall of piston plays a significant role in the

mixing and imparting turbulence, we are more interested in

finding out how it affects the whole process which

undoubtedly results in these types of configurations.

Simulations are performed and then a comparative study

between all those shapes is done. Parameters such as

incylinder-pressure, rate of heat release, TKE, temp, NO,

SOOT will be compared and analysed.

2. CFD MODELLING

CFD model has been prepared by means of AVL FIRE

code. This is finite volume based solver. An inbuilt feature

of mesh generation is employed for meshing purpose. All

the shapes are symmetrical about the vertical axis. Also

there are four nozzles for fuel entry to chamber. So it is

beneficial in terms of computational effort to split the

control volume in four parts for analysis purpose. So the

analysis will be done on 90 deg cut out of total 360 control

volume. After simulation will be done AVL FIRE has

feature of integrating the results of single cut to the whole

control volume.

Now we will be proceeding with simulation of our baseline

shape of hemispherical piston. The first step in this direction

will be of meshing. Number of boundary layers to be used is

2 with average cell size of 0.75 mm. This gives a total

number of 32,184 cells in mesh. But we also needed to be

confident about the mesh independency of results. For the

same purpose we have conducted mesh independency test

for few of parameters. This gives us confidence over the size

and number of cells. The following table 2 shows the results

of different cell size, cell numbers with their effect on

variation on maximum pressure and max temperature.

Parameters have been taken from full load simulation.

As we can see in above table, the variation in pressure and

ROHR is below 1 % and near to 1% respectively. On further

refinement of mesh there is no significant change in the

values of parameters. So we can surely say that grid size of

0.75 mm and 32,184 cells will give reliable results. Below

figure-2 shows mesh at TDC of all configurations to be

studied.

2.1 Models Used in Simulations

Commercially available AVL FIRE code has been used for

simulation. This code has been developed on finite volume

approach and pressure based segregated solution algorithm.

The SIMPLE (semi-implicit) algorithm is used for pressure-

velocity coupled equations in the solution procedure.

Calculation of boundary value is done by extrapolating

method. The derivatives have been calculated by least

square methodology. The turbulence and turbulent wall heat

transfer is modelled by K-zeta-f model given by

Hanjelic(17). Under relaxation factors used for different

parameters are around 0.9 for all. Differencing scheme used

for continuity equation is central differencing while for

momentum MINMOD scheme has been implemented. The

remaining equation of energy, turbulence and scalar

quantities has been differenced by UPWIND scheme (18).

Combustion phenomenon has been modelled by ECFM -3Z

model(19). Well proved Extended Zeldovich and Kinetic

model have been used for NOx and SOOT respectively in

the combustion process (20,21). Speed and pressure of fuel

affects the size of droplets. Dokowicz model which is

suitable for greater Weber number applications has been

employed here for evaporation. The wall spray interaction is

predicted by walljet 1 model. The spray phenomenon is

modelled by WAVE break-up model (20).Boundary wall

layer temperature profile predictions has been carried out by

Kader`s model (22). For a clearer picture, below table

summarizes phenomenon and model used in AVL FIRE.

Studies performed earlier by a number of researchers have

validated codes and models listed in AVL. Tatschl`s study

validate diesel combustion model for multidimensional

engine simulation(23). Wieser carried out study on 3-D

combustion and heat transfer models for simulations in

diesel engine(24).Primary hybrid model for break-up studied

by Suzzi etc. has proved to be consistent for non-

evaporating and evaporating sprays(25). Again Tatschl

carried out study for validates FIRE code for combustion

and diesel mixture formation (26). This study gives us

enough confidence to rely on the simulation results given by

AVL FIRE.

Hemispherical Shape Omega Shape


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3. VALIDATION OF NUMERICALMODEL

For comparative study between different configurations

firstly we will need to validate our CFD model with

experimental results. Experimental results were available for

our baseline shape of hemispherical piston as mentioned

above. As a general trend observed in other studies(16,28),

in-cylinder pressure and rate of heat release profile should

match with that of simulation results for validation purposes.

A part of compression and expansion near the vicinity of

TDC has been compared here (16) since the combustion

phenomenon is predominant near TDC. In this study we will

be validating pressure and heat release rate for both full load

and half load conditions. Fig 3 and fig 4 shows comparison

of pressure and heat release rate for full as well as half load

conditions plotted against CA near vicinity of TDC.TDC is

at 0 deg CA .The overlaid profiles matches closely giving us

enough confidence about the numerical model along with

initial and boundary conditions.

4. RESULTS AND DISCUSSIONS

Simulations were carried out for the selected piston shapes

at full load. In all the simulations, parameters other than

piston shape were kept constant. Hence compression ratio,

initial conditions, computational models, solver

methodology etc. were same at all different configurations.

The cavity volume and meshing is dependent on the piston

shape hence a little difference were observed. In-cylinder

properties such as pressure, rate of heat release, velocity

field and turbulence kinetic energy has been studied,

compared and analysed for all configurations at full load. On

the emission and combustion side, temperature, NO and

SOOT has been taken into account.

4.1 In-Cylinder Pressure

Fig 5(a)and fig 5(b)shows the pressure profile comparison of

all the configurations over a range of 60degCA BTDC to

60deg CA ATDC at full and half loading conditions. For the

conventional piston shapes of hemispherical, omega and

single curved the pressure is showing consistency

throughout the range of CA considered at both loading

condition. The peak pressure variation is under 2%.

However the double curved shaped piston at full load is

showing the slight variation in the profile till the TDC

giving higher pressure. The partition wall between two

curvatures may be attributed for this behaviour, giving a

little bit more compression effect to the air. But at half load

condition the peak is delayed by 1 deg. This should be due

to increased delay period. The peak pressure for double

curved configuration at full load is less by nearly 4% than

that of hemispherical one. All the profiles are showing

consistency after 5 deg CA ATDC at full load. The peaks for

all different configurations at half load are less than that at

full load which is general trend for pressure. The little effect

on pressure is reasoned by the small size of engine, which is

not giving much variation in pressure. The minimum peak of

double curved shape is reasoned by slightly more cavity

volume for same compression ratio.

4.2 Rate of Heat Release

Comparing rate of heat release gives us a good idea of what

is happening to the combustion in cylinder. Fig 6(a)&fig

6(b) represents the heat release rate for all configuration

plotted against CA of 10 deg BTDC to 20 deg ATDC for

full and half loading conditions. For full load conditions, the

profile for omega, hemispherical and single curved is

showing similar trend. Ignition delay period and premixed

combustion is similar. The peaks of all HRR are under 2-3%

variation. However in the double curved configuration

ignition delay period has been increased by 3 deg. This may

be attributed to less temperature of fuel which is confirmed

by temperature comparison. After dropping for a 2 deg, it is

showing consistency over the next 3 deg giving a wider

spread of profile. This is reasoned by combustion of any

remaining mixture which happens to be escaped due to

geometry. At half load a similar trend of profile is observed.

For double curved, ignition delay period is 2 deg more than

that of others. There is a sharp fall in profile of omega

shaped piston. This can be reasoned with ignition happening

near the cylinder wall giving a sudden fall in value. At half

loading condition, we can see a greater difference for

hemispherical and double curved profile, which varies from

that at full loading condition. But as we know engine gives

better performance at full load and hence full load results are

more trustworthy.

4.3 Velocity Field

The fig-7 shows the velocity distribution at various CA

ATDC. It shows the effect of piston shape. For omega and

single curved shaped piston we can see a vortex forming.

And in double curved shape, two vortexes are in a position

to form. But the intensity is showing much variation in

strength of vortexes. This is the main intention behind

selecting this type of shape for study. However in this study

we are not considering the variation of injection angle.

So it can be depicted that, for our set-up conditions inner

half of the shape is not fully used. In the hemispherical

shape, even though there can be seen formation of vortex, it

is not able to impart its energy due to absence of wall near to

it. The maximum velocity can be depicted for omega shaped

piston.

4.4 Turbulence Kinetic Energy (TKE)

Fig8(a)& fig 8(b) shows comparison of mean TKE over CA

at full and half load respectively. At full load, as we can see,

omega shape is having maximum value of the TKE out of

all configurations considered, for the current set-up. This

justifies above results of in-cylinder pressure and heat

release rate. Initially, TKE of double curved was expected to

give more turbulence and hence proper fuel-air mixture

mixing leading to effective combustion. But for the current

set-up it is showing lesser value. The reason is attributed to

the angle of fuel injection as can be seen in velocity field.

Comparing between omega and single curved, it can be

observed from velocity field that intensity of velocity for

single curved is somehow more than omega. But the overall

spread is more for omega. This reason is attributed to higher

Fig 4-a


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TKE of omega than single curved shape. At half load, we

observe a similar profile of TKE. However for omega

shaped piston we are getting a slightly higher TKE. This

should be due to the less amount of fuel giving a lighter air-

fuel mixture than at full load and hence more TKE. But for

single and double curved profile we are getting reduced

TKE at half load than at full load. Since TKE is more

sensitive to the RPM of engine we are not able to see a

correctly reasoned trend among the profiles.

4.5 Temperature

Fig-9 shows variation of mean temperature over CA. It is

showing highest temperature for omega shaped out of all.

This confirms previous results of pressure, heat release rate

and TKE. Double curved is having minimum temperature

compared to others. There is a slight suppression of 1 deg

temperature near TDC for double curved shape. It may be

due to delayed ignition of fuel air mixture. Also the spread

of temperature profile is greater just like heat release rate

and then there is a sudden drop at 40 deg ATDC. The shoot

up of profile for hemispherical shape may be justified by

sudden burning of unburned air-fuel mixture. Fig-10 shows

pictorial distribution of temperature field. It gives a realistic

view of combustion process which is happening inside the

combustion chamber. This gives another confirmation to all

the results discussed earlier. The inside curvature of double

curved shape is not utilised to full extent. Direction of fuel

injection is the reason behind this particular behaviour.

4.6 Emissions: SOOT and NO

Fig 11(b)and fig 11(d)shows the NO content plotted against

CA at full and half loading condition. As we know high

temperature is responsible for NO formation, we are getting

highest NO content in omega for full load. Formation of NO

is affected by highest temperature, reaction time and oxygen

content. The sharp increase in the NO is attributed to sudden

rise in combustion throughout the cavity. Comparing

percentage wise with maximum NO for omega shape, at full

load, we are getting 12.34 %, 15.78% and 34.87% less NO

for hemispherical, single curved and double curved shaped

piston. Double curved piston shape is showing excellent NO

characteristics for the current set-up. Compared to our

baseline shape of hemispherical piston it is showing nearly

27% less NO. On the other hand, we have with us NO

content at half load. This shows lower values of NO for all

configurations. Comparing percentagewise with max NO of

omega at half load, we see a drop of 9 % , 11 % and 40% for

hemispherical, single curved and double curved

respectively. Double curved shape is showing 33% lower

NO as compared with hemispherical shape. This drop shows

a similar trend as seen at full load.

Fig 11(a)and fig 11(c)shows SOOT plotted against CA at

full and half loading conditions respectively. SOOT is

affected by combined effect of mixing of fuel, temperature

of air-fuel mixture, oxidation level combustion. For full

load, minimum is SOOT for omega shape which is justified

by complete combustion of air-fuel mixture due to higher

TKE and temperature. A variation in the trend has been

observed for hemispherical bowl. The increased SOOT

content can be attributed to less turbulence and bigger cavity

volume which is giving residual space for particles. Less

oxidation in the mixed and late combustion phase can be

reasoned for this increased SOOT. Comparing percentage

wise with hemispherical at full load which is also our base

shape, we are getting 8%, 15.5% and 19.8% lesser SOOT

for double curved, single curved and omega shaped piston

geometry. At the half load conditions we see reduced values

of SOOT. Percentagewise comparison shows that we are

getting 15%, 13% and 10% less SOOT for omega, single

curved and double curved shape respectively. Here we also

observed a wider shape of profile for single curved as

compared to that at full load. This should be due to fall in

combustion reaction rate giving sudden rise in un-burnt air-

fuel mixture.

4.7 Efficiency and BSFC

Fig 12 shows comparison of efficiency and break specific

fuel consumption at half and full load for all configurations.

As we all know that full load efficiency of diesel engine is

better than that at part load, here it has again been

confirmed. Omega shaped piston is showing maximum

efficiency of 40 % which is consistent with many of

previous results like pressure, heat release, temperature. The

variation in efficiency is though little at part and full load.

But the variation gets reflected on BSFC. BSFC is a

performance parameter which tells the effectiveness of

diesel engine to utilise chemical energy of fuel into

combustion. Lower the BSFC more efficient the engine is.

Since for every unit of power generated we are using less

fuel. Confirming to previous trends omega shape is giving

least BSFC. It is a well-known fact that engine performs

better at full load and hence we are getting less BSFC at full

load than at half load. Fuel is not utilised properly at part

load conditions. On comparing with hemispherical shaped

piston, we got 9 % less, 1 % more and 6% more BSFC for

omega, single and double curved shape respectively. The

maximum fuel consumption is for double curved shape. It

can be reasoned by the bigger cavity volume which gives

larger surface for settlement of fuel which remains un-burnt.

However, on combining with optimum fuel injection angle it

may produce better results, which can further be explored.

5. CONCLUSION

This study aims to explore the effects of piston bowl

geometry on combustion, emission and performance. This is

a comparative study of four different piston shapes on

parameters as described above. Following are the

concluding remarks on the results.

For the current set-up, OMG shape gives the maximum

pressure while DC shape gives the minimum pressure.

However the difference of pressure is not more than6%.

On the other hand with HRR we found that OMG gives

maximum HRR while SC gives minimum. The ignition

delay of 3 deg has been observed with DC shape. So it

has been concluded that with given set-up conditions

OMG is optimum profile for pressure and HRR.


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Phenomenon of combustion is strongly dependent on

TKE. On comparing TKE we found OMG is best suited

for current configuration. DC shaped showed least TKE

which was our imaginative shape. It was predicted that

the double curvature would increase TKE resulting in the

better combustion but results were contradictory.

However variation of spray angle could result in better

results for the same shape which can further be explored.

HEMI and SC shape have shown intermediate results

between the previous two.

To have a better understanding of inside flow, in this

study we have studies velocity vectors and temperature

distribution. The results showed that because of the

partition wall in DC shape, at current angle of spray,

optimum mixing is not possible. So it can be concluded

that mixing is dependent not alone on curvature but also

the height of separating wall and spray angle.

On emissions side NO and SOOT have been compared.

OMG shape gave minimum SOOT but maximum NO on

other hand our baseline shape of HEMI gave maximum

SOOT. DC shape gave least NO emissions but to less

temperature. Hence it can be conclude that OMG is

optimum for SOOT emissions and to improve NO

emission performance other options like blending of fuel

can be explored.

Efficiency and BSFC have been compared to check

performance. OMG shape found out to be best performer

for both parameters.

On a concluding note, it can be said that OMG is the

optimum shape for the current study. It showed better

TKE results with good velocity vectors. The higher

temperature resulted in reduced SOOT but increased

NO. And the efficiency and BSFC is best among all.

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Fig 1:- Schematic Shapes of Piston


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Fig 2:- Mesh at TDC


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Fig 7: Velocity Distribution


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Fig 10: Temperature Distribution


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Fig 12: Efficiency and BSFC

Table 1: Engine Specifications

General Specs Kirloskar (Model -TV1) , Four strokes, compression ignition, Constant

Speed vertical, Water cooled, direct injection

Number of Cylinder 4

Bore X Stroke 87.5 mm X 110 mm

Rated output 5.2 kW

Compression ratio 17.5: 1

Rated speed 1500 rpm

Piston Bowl Hemispherical

Number of injectors 4

Table 2: Mesh Independency Test

Cell Size Cell numbers Max Pressure(Bar) % Variation Max HRR(J/deg) % Variation

1 mm 20,568 58.91 -1.88 38.12 -8.23

0.9 mm 23,896 59.09 -1.58 38.96 -6.21

0.8 mm 29,324 59.45 -0.98 39.56 -4.77

0.75 mm 32,184 60.04(Baseline) 0 41.54 0

0.7 mm 36,612 60.06 +0.03 42.02 +1.15

0.65 mm 40,440 60.07 +0.033 42.10 +1.34

Table 3 Model Summary

Phenomenon to be

modelled

Model

Breakup WAVE

Wall interaction Walljet1

Turbulence K-zeta-f

Evaporation Dukowics

NO Extended Zeldovich

SOOT Kinetic Model

EFFECTS OF PISTON BOWL GEOMETRY ON COMBUSTION AND … · 2017-11-03 · Abstract The present paper...

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